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Article

Effect of Silane-Modified Ammonium Polyphosphate on the Mechanical, Thermal, and Flame-Retardant Properties of Rice Husk/Polylactic Acid Composites

by
Yufeng Sun
1,2,*,
Mingyang Liu
1,
Ziheng Zhang
1,
Hengyu Liu
1,
Dongming Shi
1,
Jilai Ying
1,
Wenlong Mu
1,
Defeng Li
1 and
Ing Kong
2,*
1
College of Mechanical and Electrical Engineering, Henan Agricultural University, Zhengzhou 450002, China
2
Advanced Polymer and Composite Materials Laboratory, Department of Engineering, School of Computing, Engineering and Mathematical Sciences, La Trobe University, Bendigo, VIC 3550, Australia
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(5), 251; https://doi.org/10.3390/jcs9050251
Submission received: 24 April 2025 / Revised: 13 May 2025 / Accepted: 15 May 2025 / Published: 19 May 2025
(This article belongs to the Section Composites Modelling and Characterization)
Browse Figures
Versions Notes

Abstract

:
In this study, rice husk (RH, 15wt%) served as a carbonizing agent, and ammonium polyphosphate (APP) served as an acid and gas source. These were combined with polylactic acid (PLA) to develop a high-strength and flame-retardant PLA-based composite. The APP surface was modified with silane coupling agents (KH550 and KH570) to enhance the compatibility with the PLA matrix and improve both mechanical and flame-retardant properties. The composite was evaluated using UL-94 flame retardancy tests, limiting oxygen index (LOI) measurements, and mechanical properties assessments. The findings demonstrated that both PLA/RH-APP10% and PLA/RH-APP15% composites met the UL-94 V-0 standard. Increasing APP content enhanced flame retardancy but reduced mechanical strength. Compared to unmodified PLA composite, the PLA/KAPP5% composite exhibited an 18.7% increase in tensile strength, an elongation at break improvement from 3.26% to 4.09%, and a LOI of 27.9%. The silane modification significantly improved APP dispersion within the PLA matrix, increasing interfacial contact and improving overall mechanical properties. The flame retardancy improvements were attributed to reduced thermal decomposition rates and increased carbon residue formation.

Graphical Abstract

1. Introduction

Amid the global plastic pollution crisis, biodegradable plastics such as polylactic acid (PLA) have gained popularity due to their renewable origin and eventual biodegradability in natural environments [1]. PLA exhibits excellent mechanical properties and processing versatility [2], making it suitable for applications in electrical and transportation engineering [3,4]. However, its inherent flammability and dripping behavior during combustion fail to meet safety standards for automotive and electronic applications [5,6,7]. To address this, the researchers have explored various flame-retardant strategies to enhance PLA’s fire resistance. Recent advancements include the incorporation of nanoparticles [8,9,10], metal hydroxide composite systems [11], and expandable flame retardants [12].
Intumescent flame retardants (IFRs) have emerged as a focal point within the realm of materials science due to their distinctive char-forming mechanism, which provides exceptional flame retardancy. The triad of components in IFRs—an acid source, a carbon source, and a gas source—interact synergistically under elevated temperatures to form an insulating, porous carbonaceous layer. This layer effectively impedes the transfer of oxygen and heat, thereby achieving a pronounced flame-retardant effect [13,14,15]. Ammonium polyphosphate (APP), a quintessential IFR component, has demonstrated remarkable efficacy in the context of combustion processes [16]. With high phosphate and nitrogen content, APP possesses the dual functionality of an acid and gas source [17]. It is non-toxic and produces minimal smoke emissions, making it a promising flame retardant with broad applications. However, its practical use presents challenges, as its poor compatibility with polymer matrices can compromise the mechanical integrity of composites.
To mitigate this issue, microencapsulation and surface coating techniques have emerged as effective strategies for modifying APP, thereby improving its compatibility with polymer substrates [18]. Xiao Dong et al. [19] developed an innovative intumescent flame retardant (A-A) through an ion exchange reaction between APP and aminobenzenesulfonic acid. When 15 wt% of A-A was incorporated into pure PLA, the limiting oxygen index (LOI) increased significantly from 20.0% to 30.5%, the UL-94 vertical burn test rating improved from unrated (grade 0) to the highest grade (V-0), and the heat release rate in cone calorimetry (CONE) was significantly reduced. Similarly, Meng et al. [20] synthesized a novel intumescent flame retardant (TRAPP) by combining tannic acid (TA) with polyethyleneimine (PEI) as a bridging agent, with ammonium polyphosphate (APP) as the core. This innovative formulation not only resolved the compatibility issues between PLA and APP but also greatly enhanced the flame retardancy and ultraviolet (UV) resistance of PLA. In another study, Wang et al. [21] introduced microencapsulated APP (MCAPP) into a PLA/starch bio-composite. With 30% IFR content, the composite achieved a UL-94 V-0 rating and exhibited a high LOI value of 41%, underscoring the potential of such additives in advancing the fire safety of biodegradable polymers. Collectively, these advancements have made significant progress in the development and optimization of PLA-based materials for applications in industries with stringent flammability requirements.
Due to their natural abundance and sustainability, biomass materials have recently garnered significant attention in polymer science for their potential use as additives in the development of flame-retardant polymers [22]. Notably, a variety of biomass-derived substances, including starch [21], cellulose [23], and lignin [24], have been successfully integrated into intumescent flame retardant (IFR) systems to improve the fire resistance of polymeric materials.
Concurrently, plant fibers are being explored as reinforcing agents because of their ability to improve the mechanical characteristics of polymers while also being biodegradable and environmentally friendly [25]. Among these, rice husk, an agricultural by-product valued for its low cost and ease of degradation, is primarily composed of cellulose and hemicellulose [26]. Previous studies have demonstrated that incorporating 10–20 wt% of rice husk in PLA composites can optimize mechanical reinforcement while maintaining processability. For instance, Sun et al. [27] reported that 15 wt% RH loading in PLA composites achieved a favorable balance between tensile strength (55 MPa) and elongation at break (4.2%), avoiding excessive brittleness. Similarly, Wu et al. [28] found that RH loadings above 20 wt% led to filler agglomeration and reduced interfacial adhesion, whereas loadings below 10 wt% failed to provide adequate flame-retardant synergy with APP. These findings validate our selection of 15 wt% RH in this study as an optimal composition for achieving balanced filler dispersion, interfacial bonding, and flame-retardant performance. These components, rich in hydroxyl groups, make rice husk an excellent candidate both for reinforcing polymer matrices and serving as a carbon source in flame-retardant systems.
This study explores a combined approach using rice husk powder as a carbonizing agent and ammonium polyphosphate (APP) to formulate an intumescent flame-retardant system for PLA composites. While rice husk has previously been utilized as a reinforcement in biodegradable PLA composites [28] and APP is widely recognized for enhancing the flame retardancy of PLA [29], the present work focuses on their synergistic interaction. In particular, the study examines the effect of silane-modified APP on optimizing both the flame-retardant and mechanical performance of the resulting composites. The dual objectives of this approach are to enhance the flame retardancy of polylactic acid (PLA) composites and to improve their mechanical and thermal properties. A comprehensive experimental investigation was conducted to assess the flame-retardant performance, thermal stability, and mechanical behavior of PLA composites incorporating varying proportions of the newly formulated system. The findings of this research provide experimental insights into the synergistic effects of biomass-derived rice husk and silane-modified APP in enhancing flame retardancy of PLA composites, offering practical guidance for optimizing such systems. Although quantum chemical analyses (e.g., NBO) were not conducted, the observed physicochemical interactions, such as enhanced char formation and gas-phase radical quenching, align with established mechanisms of intumescent flame retardants [30,31]. This study provides experimental insights into the synergistic effects of biomass-derived rice husk and silane-modified ammonium polyphosphate (APP) in enhancing the flame retardancy of PLA composites.

2. Materials and Methods

2.1. Materials

Polylactic acid (PLA, grade 4032D) was acquired from NatureWorks LLC (Plymouth, MN, USA). Ammonium polyphosphate (APP, Type I), with a phosphorus content of ≥31% and a molecular formula of (NH4PO3)n (n > 1000), was supplied by Dongwan Xingyuan Chemical Co., Ltd. (Dongguan, China). Acetic acid (AR grade) was obtained from Sinophosphate Chemical Reagent Co., Ltd. (Shanghai, China). Rice husk powder (200 mesh) was purchased from Lianyungang Surui Straw Processing Plant (Liangyungang, China). Silane coupling agents KH550 (γ-aminopropyltriethoxysilane) and KH570 (γ-methacryloxypropyltrimethoxysilane) were supplied by Nanjing Shuguang Group Co., Ltd. (Nanjing, China). Rice husk powder was added at 15 wt% based on preliminary optimization trials [26,27,28], which showed that higher loadings (>20 wt%) led to particle agglomeration and reduced mechanical properties, while lower loadings (<10 wt%) failed to achieve sufficient flame-retardant synergy with APP. This intermediate loading was selected to ensure effective dispersion, strong interfacial adhesion, and balanced flame-retardant performance.

2.2. Preparation of Modified APP

The modification mechanism of APP with KH550 is illustrated in Figure 1. Initially, the silane coupling agent was mixed with deionized water and anhydrous ethanol in a mass ratio of 1:3:6. Acetic acid was added to adjust the pH of the mixture to 4. The reaction was conducted at 60 °C for 2 h to ensure complete hydrolysis of the silane coupling agent, resulting in a mixed solvent system containing the hydrolysis products. Then, 100 g of type I APP was dispersed in 150 ml of anhydrous ethanol, again at 60 °C under continuous stirring. The prepared hydrolyzed solution was then gradually added to the APP dispersion while stirring was maintained until the reaction was complete. The resulting mixture was cooled to room temperature, filtered, and the solid product was dried in a vacuum oven at 60 °C for 12 h. The product was further washed multiple times with ethanol to ensure purity. The final white solid product was designated as KAPP-5. Using the same procedure, KAPP-7 was also prepared.

2.3. Preparation of PLA Composites

Before fabricating the PLA composites, PLA, RH, and APP were pre-dried in an oven at 60 °C for 12 h to remove moisture. The dried materials were then melt-blended using a twin-screw extruder (SJZS-10B, Wuhan Ruiming Laboratory Instruments Co., Ltd., Wuhan, China) under controlled temperature conditions based on specific formulations. The extruded materials were subsequently molded into specimens using an injection molding machine (SZS-20, Wuhan Ruiming Laboratory Instruments Co., Ltd.). Detailed formulations and processing parameters for the PLA composites are presented in Table 1.

2.4. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared spectroscopy was conducted using an IRT racer-100 spectrometer. Approximately 1–2 mg of dried sample powder was thoroughly mixed with potassium bromide at a mass ratio of 1:100. The mixture was ground evenly in a mortar, pressed into a pellet, and then subjected to FTIR. Spectra were recorded over the range of 4000 cm−1–400 cm−1 with a resolution of 4 cm−1.

2.5. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed using an SDT 650 analyzer. The test was conducted under a nitrogen atmosphere with a flow rate of 50 mL/min. Samples were heated from 40 to 800 °C at a constant rate of 10 °C/min.

2.6. Flame Retardancy Testing

The vertical burning behavior of PLA composites was assessed using a CZF-3 horizontal and vertical combustion tester (Nanjing Jiangning Analytical Instrument Co., Ltd., Nanjing, China), following the GB/T 2408-2008 UL-94 standard [32]. The limiting oxygen index (LOI) was determined using a JF-3 oxygen index tester, in accordance with GB/T2406-2009. Test samples measured 80 mm × 10 mm × 4 mm.

2.7. Fracture Surface Analysis

To observe the fracture surface morphology, the sample was coated with conductive gold after applying conductive adhesive. The analysis was performed using a ZEISS Sigma 500 scanning electron microscope (Carl Zeiss AG, Baden-Wurttemberg, Germany) at an acceleration voltage of 20 kV under vacuum conditions.

2.8. Mechanical Properties Analysis

Mechanical performance, including tensile and flexural properties, was evaluated using a DF13.105D electronic universal testing machine. Tensile testing followed the GB/T 1040.1-2006 standard [33], using specimens of 75 mm × 10 mm × 2 mm, and a test speed of 5 mm/min. Flexural testing adhered to GB/T 9341 2000, using specimens of 80 mm × 10 mm × 4 mm at a rate of 2 mm/min. Each group was tested five times, and the average values were reported.

3. Results

3.1. Characterization of APP and Silane-Modified APP

Figure 2 displays the FTIR spectra of APP, KAPP-5, and KAPP-7. Characteristic peaks corresponding to NH4+ were observed at 3200, 3059, and 1435 cm−1 [34]. In addition, peaks at 800 cm−1 (P-O-P), 880 cm−1 (P-O asymmetric stretching), 1066 cm−1 (P-O symmetric stretching), 1014 (PO2 and PO3 symmetric vibrations), 1251 cm−1 (P=O), and 1694 cm−1 (P-OH) were also detected [20]. The FTIR spectra of both modified and unmodified APP samples are almost identical, as shown in Figure 2. The peaks in the 800–880 cm−1 region may be attributed to Si-O-C or Si-O-Si groups introduced by the silane coupling agent; however, this region also overlaps with the symmetric stretching vibration of P-O in P-O-P bonds [35]. The lack of distinct spectral differences between modified and unmodified APP samples suggests that FTIR analysis alone may be insufficient to conclusively confirm silane functionalization [36]. To resolve this ambiguity, future studies should incorporate complementary techniques such as X-ray fluorescence (XRF) or inductively coupled plasma mass spectrometry (ICP-MS) to quantify silicon (Si) content in the modified APP. An elevated Si concentration would provide direct evidence of silane coupling agent attachment, thereby validating the surface modification mechanism proposed in Figure 1.
The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of APP, KAPP-5, and KAPP-7 are compared in Figure 3, with specific degradation parameters summarized in Table 2. The decomposition process occurred primarily in two stages. The first stage began around 280 °C, during which NH3 and H2O were released as the main volatile products, accompanied by the formation of cross-linked polyphosphates (resulting in approximately 20% mass loss) [37]. The second stage, between 500 and 700 °C, represented the main degradation of APP, accounting for approximately 81% of the total mass loss [38]. The initial decomposition temperature (T5%) of APP was recorded at 300.6 °C. In contrast, the T5% values for KAPP-5 and KAPP-7 were higher, at 316.9 °C and 308.3 °C, respectively, indicating enhanced thermal stability. This improvement is attributed to the presence of silane coupling agents (KH550 and KH570), which likely improved the dispersity of APP and reduced particle aggregation, thereby minimizing localized heat accumulation and delaying thermal degradation [39]. Notably, KAPP-5 showed superior thermal stability, with a significantly lower decomposition rate at Tmax2 (606.4 °C) and a higher char residue (21.7%) compared to unmodified APP during the initial degradation stage. These findings suggest that silane modification enhances the thermal stability of APP and promotes more effective char formation at elevated temperatures. The silane coupling agents (KH550/KH570) form a protective layer on the surface of APP particles, which reduces particle aggregation and improves dispersion within the PLA matrix. This homogeneous distribution delays the onset of thermal degradation by minimizing localized heat accumulation. Additionally, the silane-modified APP releases phosphorus-rich acidic species during decomposition, which catalyze cross-linking reactions between rice husk (carbon source) and degradation products, leading to a more stable and expanded char layer [31].

3.2. Flame-Retardant Properties

Vertical combustion and LOI tests were performed on the PLA-based composite. The results are summarized in Table 3. Pure PLA exhibited a low LOI value of 20% and did not achieve a rating in the UL-94 vertical burning test [40]. Because the addition of RH, which is highly flammable and prone to degradation, slightly decreased the LOI to 19.8%, the composite still failed to achieve a UL-94 rating. This indicated that incorporating RH alone does not improve the flame-retardant properties of PLA. While RH primarily acts as a carbon source, it lacks the necessary acid or gas release to form a cohesive intumescent char. During combustion, RH undergoes rapid thermal decomposition, releasing flammable volatiles (e.g., cellulose derivatives) that counteract its carbonizing effect [41]. In the absence of APP’s phosphate and ammonia release, RH is unable to synergistically generate a protective char layer, resulting in negligible flame-retardant enhancement. However, the inclusion of APP significantly enhanced the flame-retardant performance. Compared with pure PLA [38], the LOI of APP-containing composites increased notably, and the UL-94 rating improved from “No rating” (NR) to V-0, the highest classification. As can be seen from the degree of dripping behavior of PLA composites, although the APP addition helped suppress melt dripping, it was only partially effective. When 15 wt% of APP was added, the LOI reached 27.4%, representing a 7.4% improvement over pure PLA. Furthermore, the incorporation of KAPP-5 and KAPP-7 resulted in even greater flame-retardant performance, with LOI values rising to 27.9 and 29.4, respectively, compared to 26.6% for the A10 composite. These results demonstrate that silane modification of APP can further enhance its effectiveness as a flame retardant in PLA composites.
Pure PLA burned readily and exhibited a pronounced melting and dripping phenomenon, as seen in Figure 4a. Although sample A5 showed the formation of a carbon layer after combustion, the dripping phenomenon remained evident. In contrast, sample A10 displayed a moderate intumescent effect (Figure 4c), attributed to the synergistic interaction between APP and rice husk, at an optimal ratio, which enabled the formation of an intumescent flame-retardant system. However, the carbon layer formed in A15 was significantly thinner than that of A10, which may be due to an imbalanced ratio in the intumescent system [42]. To further enhance flame retardancy, samples A10-K5 and A10-K7 were prepared by modifying APP with KH550 and KH570, respectively. These composites achieved higher LOI values of 27.9% and 29.4%, compared to 26.6% for A10, indicating improved flame-retardant performance. As shown in Figure 4f,g, both modified composites exhibited intumescence, with A10-K5 showing a more prominent expansion effect than A10-K7. This can be attributed to the rice husk’s contribution as a carbon source, facilitating the formation of a stable, expanded char layer [43]. During thermal decomposition, the gases released by APP contribute to the swelling of the matrix, resulting in a porous, insulating carbon layer [44]. This expanded char structure effectively serves as a barrier to heat and oxygen, thereby lowering both combustion temperature and burning rate of the composites.

3.3. Thermal Properties

The TG and DTG curves for various PLA-based samples are displayed in Figure 5, illustrating the thermal degradation behaviors under heating. Table 4 summarizes the key thermal parameters. Pure PLA exhibited near-complete decomposition at 600 °C, with only 1.3% char residual remaining [43]. With increasing APP content, the PLA composites exhibited reduced maximum decomposition rates (Tmax) and higher residual char content. Specifically, the incorporation of 10% APP (A10) increased the composite’s thermal stability, decreasing the Tmax decomposition rate by approximately 6% compared to pure PLA, and increasing the char residue by 8.1%. On the other hand, the inclusion of silane-modified APP (KAPP-5 and KAPP-7) also led to a reduction in the Tmax and an increase in the final char yield at 600 °C [45]. While PLA composites showed a slightly earlier onset of thermal degradation compared to pure PLA, this was attributed to two factors: (1) the inherently lower thermal decomposition temperature of rice husk, and (2) the initial release of NH3 and phosphate during the early thermal degradation of silane-modified APP. Despite the early onset, the high-temperature thermal stability of A10-K5 and A10-K7 surpassed that of pure PLA. This enhancement is attributed to the catalytic effect of silane-modified APP, which facilitated char formation and promoted the generation of thermally stable carbonaceous substances during degradation [29].

3.4. Flame-Retardant Mechanism

The flame-retardant mechanism of the A10-K5 is displayed in Figure 6. The silane-modified APP and RH acted synergistically in both the gas and condensed phases to improve the flame retardancy of PLA. From the gas phase perspective, the combustion of A10-K5 generated non-flammable gases such as CO2 and NH3, which helped dilute flammable gases and reduce their concentration [46]. Upon heating, APP decomposed into polyphosphate, ammonia, and PO∙ free radicals. These radicals effectively captured highly reactive H∙ and HO∙ radicals, interrupting the radical flame propagation cycle [47]. In the condensed phase, RH not only underwent thermal dehydration to form char but also reacted with phosphoric acid to create a cross-linked porous carbon layer [48]. The expanded carbon structure formed a protective barrier on the surface of the material. The dense carbon layer hindered the release of flammable volatiles and limited heat and oxygen transfer. Furthermore, the formation of a dense carbon layer and cross-linked structure during combustion physically hindered the release of flammable volatiles, effectively suppressing dripping. While melt viscosity changes were not quantitatively measured in this study, the observed reduction in dripping is consistent with previous research, where the formation of char has been shown to correlate with increased melt viscosity in flame-retardant PLA systems [31,41]. As a result, the thermal and flame-retardant performance of the composite was significantly improved.

3.5. Morphology

Figure 7 depicts the tensile fracture morphologies of the PLA composites. Pure PLA exhibited a smooth fracture surface characteristic of a typical brittle fracture mode. In sample A5 (Figure 7b), the incorporation of 5% APP resulted in uniform dispersion without visible agglomeration, indicating good compatibility at this filler level. However, as the APP content increased, interfacial adhesion between the RH fiber and the PLA matrix weakened. This was evident in A10 (Figure 7c) and A15 (Figure 7d), where voids were observed around APP particles. In A15, the voids were larger, and several RH fibers were visibly pulled out and detached from the PLA matrix. This behavior is attributed to poor interfacial bonding, which led to debonding between the APP particles and the PLA matrix during the tensile loading. The increased void content and poor interfacial adhesion caused a reduction in yield tensile strength and elongation at break, as the PLA matrix fractured under relatively high stress. In contrast, both A10-K5 (Figure 7e) and A10-K7 (Figure 7f), which were treated with silane coupling agents, showed smooth, brittle fracture surfaces without prominent voids. These observations suggest improved interfacial compatibility between APP and PLA due to silane treatment, resulting in enhanced dispersion and stronger interfacial bonding. Consequently, the modified APP contributed to improved mechanical integrity and tensile performance of the composites.

3.6. Mechanical Property

Tensile tests were conducted to assess the influence of APP content and silane-modified APP addition on the mechanical characteristics of PLA composites. The tensile findings are presented in Table 5 and Figure 8. Pure PLA exhibited typical brittle fracture behavior, with a tensile strength of 62.03 MPa and an elongation at break of 3.72%. The addition of APP significantly reduced the tensile strength and elongation at break of the PLA composites. Among the APP-containing samples, A15 exhibited the lowest tensile strength (43.91 MPa), Young’s modulus (1.99 GPa), and elongation at break (3.08%). While A5 and A10 exhibited slight differences in tensile strength and modulus, the variations were not statistically significant. Considering flame retardancy performance [49], A10 was selected for further modification. After the incorporation of silane-modified APP, the mechanical properties of A10-K5 and A10-K7 were significantly improved. The tensile strengths of A10-K5 and A10-K7 were 59.92 MPa and 56.21 MPa, respectively. Although changes in the elastic modulus were minimal, the elongation at break improved to 3.93% for A10-K5 and 4.09% for A10-K7. These enhancements can be attributed to the improved compatibility and interfacial bonding between the modified APP and the PLA matrix. Silane coupling agents (KH550/KH570) chemically graft onto the surface of APP, creating hydroxyl groups that form hydrogen bonds with the ester groups of PLA [50]. This stronger interaction reduces interfacial voids (as seen in Figure 7e,f) and enhances stress transfer efficiency, leading to higher tensile strength and elongation at break.

4. Conclusions

In this work, a flame-retardant PLA composite was developed using rice husk, a biodegradable filler, combined with APP as a carbonizing agent. The results demonstrated that the incorporation of RH and APP significantly enhanced both the mechanical and flame-retardant properties of the PLA matrix. Samples A10 and A15 achieved the UL-94 V-0 flame-retardant rating, with flame-retardant performance improving progressively with an increasing APP content. It is noteworthy that the use of silane-modified APP led to a reduction in the decomposition rate at the maximum degradation temperature (Tmax) and increased the LOI of the PLA composite to 29.4%, suggesting superior flame-retardant efficacy compared to unmodified APP. Mechanical testing revealed that while unmodified APP reduced the tensile strength and elongation at break of PLA, the introduction of silane-modified APP (KAPP-5 and KAPP-7) significantly enhanced filler–matrix compatibility. This enhancement resulted in the recovery of tensile strength and an increase in elongation at break, indicating better interfacial adhesion.
Additionally, the flame-retardant mechanism of the modified PLA composites was elucidated. The synergistic impact of silane-modified APP and RH was evident in both the gas phase and condensed phases, promoting the release of non-flammable gases and the formation of an expanded carbonaceous layer, enhancing flame retardancy.
In conclusion, this work demonstrates the synergistic efficacy of silane-modified APP and rice husk in enhancing PLA composites. The key achievements include the following:
  • Flame retardancy: PLA/RH-APP15% and PLA/KAPP-7 achieved UL-94 V-0 ratings with LOI values up to 29.4%, outperforming unmodified APP systems.
  • Mechanical performance: Silane modification restored tensile strength to 59.92 MPa (A10-K5), mitigating the strength loss observed with unmodified APP.
  • Thermal stability: Silane-modified APP increased char residue by 21.7% (KAPP-5) and delayed decomposition by 16.3 °C compared to unmodified APP.
In conclusion, this work demonstrates an optimized approach to enhance the flame retardancy and mechanical properties of PLA composites by integrating rice husk and silane-modified APP. The findings build upon existing knowledge on the individual contributions of these components, highlighting their synergistic effect within an intumescent flame-retardant system. Although standardized biodegradability tests were not conducted in this study, the incorporation of PLA and rice husk, both inherently biodegradable components, suggests a potentially reduced environmental footprint compared to conventional petroleum-based composites. Future work will include controlled soil-based biodegradation assessments to validate these assumptions and further explore the environmental performance of the developed system.

Author Contributions

Y.S.: Funding acquisition (lead); resources (lead); writing—original draft (lead). M.L.: Software (lead); writing—original draft (equal). Z.Z.: Formal analysis (lead); writing—review (equal). H.L.: Writing—original draft (supporting); writing—review and editing (supporting). D.S.: Experiment (leading). J.Y.: Writing—original draft (supporting); Resources (supporting). W.M.: writing—review and editing (supporting). D.L.: Experiment (supporting); Resources (supporting). I.K.: Final review and editing (lead). All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the China Scholarship Council (Grant No. 202408410239) and the Henan Scientific and Technological Research Program (Grant No. 182102110027).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
RHRice husk
APPAmmonium polyphosphate
PLAPolylactic acid
LOILimiting oxygen index
IFRsIntumescent flame retardants
CONECone calorimetry
TRAPPNovel intumescent flame retardant
TATannic acid
PEIPolyethyleneimine
UVUltraviolet
MCAPPMicroencapsulated APP
FTIRFourier transform infrared spectroscopy
TGAThermogravimetric analysis
TGThermogravimetric
DTGDerivative thermogravimetric
KAPP-5KH550 modified APP
KAPP-7KH570 modified APP

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Figure 1. Schematic illustration of the surface medication mechanism of APP with KH550.
Figure 1. Schematic illustration of the surface medication mechanism of APP with KH550.
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Figure 2. FTIR spectra of APP and APP treated with KH550 and KH570.
Figure 2. FTIR spectra of APP and APP treated with KH550 and KH570.
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Figure 3. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of APP, KAPP-5, and KAPP-7.
Figure 3. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of APP, KAPP-5, and KAPP-7.
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Figure 4. Digital images of PLA-based composites after the LOI test: (a) PLA; (b) A0; (c) A5; (d) A10; (e) A15; (f) A10-K5; (g) A10-K7.
Figure 4. Digital images of PLA-based composites after the LOI test: (a) PLA; (b) A0; (c) A5; (d) A10; (e) A15; (f) A10-K5; (g) A10-K7.
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Figure 5. TG curves and DTG curves of neat PLA and PLA-based composites.
Figure 5. TG curves and DTG curves of neat PLA and PLA-based composites.
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Figure 6. Schematic illustration of the flame-retardant mechanism of A10-K5.
Figure 6. Schematic illustration of the flame-retardant mechanism of A10-K5.
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Figure 7. SEM images of tensile fracture surfaces of PLA composites: (a) A0; (b) A5; (c) A10; (d) A15; (e) A10-5; (f) A10-7.
Figure 7. SEM images of tensile fracture surfaces of PLA composites: (a) A0; (b) A5; (c) A10; (d) A15; (e) A10-5; (f) A10-7.
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Figure 8. Mechanical properties of PLA composites.
Figure 8. Mechanical properties of PLA composites.
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Table 1. Formulations of PLA composites.
Table 1. Formulations of PLA composites.
SamplePLA/%APP/%RH/%KAPP-5/%KAPP-7/%
PLA1000000
A08501500
A58051500
A1075101500
A1570151500
A10-K575015100
A10-K775015010
Table 2. Thermal degradation data of APP, KAPP-5, and KAPP-7.
Table 2. Thermal degradation data of APP, KAPP-5, and KAPP-7.
SampleT5%Tmax1Mass Loss
Rate at Tmax1		Tmax2Mass Loss
Rate at Tmax2		Residual at
800 °C
°C°Cwt%∙min−1°Cwt%∙min−1%
APP300.6315.31.8592.612.418.9
KAPP-5316.9323.51.6606.49.721.7
KAPP-7308.3327.41.7590.59.823.4
Table 3. LOI and UL-94 results of PLA composites.
Table 3. LOI and UL-94 results of PLA composites.
SampleLOI (%)UL-94
DrippingIgnition of
Cotton		Rating
PLA20Heavy DrippingYesNR
A019.8Heavy DrippingYesNR
A523.3DrippingYesV-2
A1026.6Slight DrippingNoV-0
A1527.4Slight DrippingNoV-0
A10-K527.9Slight DrippingNoV-0
A10-K729.4Significantly ReducedNoV-0
Table 4. Thermal degradation data for PLA and PLA-based composites.
Table 4. Thermal degradation data for PLA and PLA-based composites.
SampleT5%TmaxMass Loss
Rate at Tmax		Residual at
600 °C
°C°Cwt%∙min−1%
PLA311.0352.120.81.3
A0298.6357.416.85.8
A5284.3363.015.79.8
A10279.0357.514.89.4
A15291.6351.613.017.6
A10-K5291.0357.413.912.7
A10-K7291.1357.514.311.5
Table 5. Mechanical properties of PLA composites.
Table 5. Mechanical properties of PLA composites.
CompositesTensile
Strength (MPa)		Young’s
Modulus (GPa)		Elongation at
Break (%)
PLA62.03 ± 0.383.1 ± 0.033.72 ± 0.11
A055.43 ± 0.622.50 ± 0.263.20 ± 0.38
A551.56 ± 1.062.11 ± 0.183.28 ± 0.37
A1050.46 ± 1.012.15 ± 0.093.26 ± 0.19
A1543.91 ± 0.451.99 ± 0.103.08 ± 0.40
A10-K559.92 ± 0.422.06 ± 0.124.09 ± 0.38
A10-K756.21 ± 0.682.05 ± 0.123.93 ± 0.57
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Sun, Y.; Liu, M.; Zhang, Z.; Liu, H.; Shi, D.; Ying, J.; Mu, W.; Li, D.; Kong, I. Effect of Silane-Modified Ammonium Polyphosphate on the Mechanical, Thermal, and Flame-Retardant Properties of Rice Husk/Polylactic Acid Composites. J. Compos. Sci. 2025, 9, 251. https://doi.org/10.3390/jcs9050251

AMA Style

Sun Y, Liu M, Zhang Z, Liu H, Shi D, Ying J, Mu W, Li D, Kong I. Effect of Silane-Modified Ammonium Polyphosphate on the Mechanical, Thermal, and Flame-Retardant Properties of Rice Husk/Polylactic Acid Composites. Journal of Composites Science. 2025; 9(5):251. https://doi.org/10.3390/jcs9050251

Chicago/Turabian Style

Sun, Yufeng, Mingyang Liu, Ziheng Zhang, Hengyu Liu, Dongming Shi, Jilai Ying, Wenlong Mu, Defeng Li, and Ing Kong. 2025. "Effect of Silane-Modified Ammonium Polyphosphate on the Mechanical, Thermal, and Flame-Retardant Properties of Rice Husk/Polylactic Acid Composites" Journal of Composites Science 9, no. 5: 251. https://doi.org/10.3390/jcs9050251

APA Style

Sun, Y., Liu, M., Zhang, Z., Liu, H., Shi, D., Ying, J., Mu, W., Li, D., & Kong, I. (2025). Effect of Silane-Modified Ammonium Polyphosphate on the Mechanical, Thermal, and Flame-Retardant Properties of Rice Husk/Polylactic Acid Composites. Journal of Composites Science, 9(5), 251. https://doi.org/10.3390/jcs9050251

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